An external structure of a sample can be observed by focusing a beam of electrons through an electromagnetic field lens, scanning and irradiating a sample with the focused beam of electrons, and detecting secondary electron charged particles emitted from the sample. This is called a scanning electron microscope. On the other hand, an external structure of a sample can also be observed by focusing a beam of ions through an electromagnetic field lens, scanning and irradiating a sample with the beam of ions, and detecting secondary charged particles emitted from the sample. This is called a Scanning Ion Microscope (hereinafter abbreviated to SIM).
As an ion source for use in a SIM, it is preferable to use a Gas Field Ionization Source (hereinafter abbreviated to GFIS).
In a SIM using the GFIS, in order to obtain a sample image with reduced noise, it is necessary to obtain an ion beam having a large current density on a sample. To do so, it is necessary to increase an ion emission angle current density of the field ionization source. To increase the ion emission angle current density, it will be expedient to increase the density of ionized gas near an emitter tip.
Then, by cooling the temperature of the emitter tip to a low temperature, molecules of ionized gas hitting against the emitter tip become massed together, as their energy decreases, and, therefore, the density of the molecules of ionized gas can be increased. As means for cooling the emitter tip, a mechanical refrigerator is preferable.
As a matter of course, the density of ionized gas near the emitter tip can be increased by increasing the pressure of ionized gas introduced into a space around the emitter tip. However, the gas introduction at 1 Pa or more causes a problem in which the ion beam neutralizes by colliding with the ionized gas and the ion beam current decreases or a glow discharge occurs. To solve this problem, it is known that the ion emission angle current density is increased by narrowing a gas ionization region by forming a projection tip with several atoms at the extremity of the emitter tip and by efficient ionization of ionized gas that is supplied in a limited quantity.
Patent Literature 1 discloses an example of a charged particle emission gun in which cleaning a tip can be performed without deactivating the charged particle emission gun for a long time and without heating the tip.